Windy Hills

Considered one of Delaware's most famous Piedmont outcrops, the Windy Hills Bridge outcrop is composed of mafic and felsic gneiss of the Windy Hills Gneiss. Much of the layering in the outcrop is regular and is 8 to 10" thick. At the contact between these layers there is evidence of partial melting. In terms of mineralogy, this rock contains mainly hornblende, plagioclase, quartz, biotite and epidote. This outcrop shows tight folds that plunge steeply 70-90 degrees to the northeast and southwest. The gneiss is cut by a long lens of pegmatite, which intruded after the folding and metamorphosing that yielded the gneiss.

There is also an interesting layer of cobble just above the bedrock in this area presumed to be the contact with the Coastal Plain sediments. These newer outcrops to the southwest display a 4-10" pelitic layer which becomes more extremely magmatic, with 1" leucosomes and ½" mafic selvages.

Overall, strikes of foliations of the mafic and felsic layering in these outcrops are 70-75 degrees east of north and the dips are a steep 80-85 degrees to the southeast, or almost vertical.

This report accompanies a new map that revises the original bedrock geologic maps of the Delaware Piedmont compiled by Woodruff and Thompson and published by the Delaware Geological Survey (DGS) in 1972 and 1975. Combined detailed mapping, petrography, geochemistry, and U-Pb geochronology have allowed us to redefine two rock units and formally recognize eleven new units. A section of the Pennsylvania Piedmont is included on the new map to show the entire extent of the Mill Creek Nappe and the Arden Plutonic Supersuite.